Totally symmetric order parameter in the framework of the phenomenological theory of phase transitions: Ferroelastics
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RATURE SOLID-STATE PHYSICS
Totally Symmetric Order Parameter in the Framework of the Phenomenological Theory of Phase Transitions: Ferroelastics A. Yu. Gufan Research Institute of Physics, Rostov State University, pr. Stachki 194, Rostov-on-Don, 344002 Russia e-mail: [email protected], [email protected] Abstract—A new approach is proposed for constructing the phenomenological theory of phase transitions. The approach is based on the classical Landau theory with allowance made for the order parameter that corresponds to changes in the charge distribution probability density of a crystal and does not affect the symmetry of the high-symmetry phase. It is demonstrated that this approach makes it possible to describe phase transitions in terms of a nonequilibrium polynomial Landau potential of degree four in the components of the order parameters. The capabilities of the proposed approach are illustrated with three systems that undergo ferroelastic phase transitions. PACS numbers: 61.50.Ks, 61.50.Ah DOI: 10.1134/S1063776107070266
1. INTRODUCTION The Landau theory of second-order phase transitions has been widely used in the physics of the solid state. This theory has provided the basis for the explanation of a great diversity of specific features observed in the properties accompanying phase transitions. These are jumps in heat capacity, the ordered state of solid solutions and alloys, superconductivity, ordering of dipole moments in ferroelectrics, jumps in elastic moduli, and the state of antiferromagnets with a weak ferromagnetism. A number of remarkable effects characteristic of materials in the ordered state, such as the linear (in the magnetic field) piezomagnetic effect in antiferromagnets and the linear magnetoelectric effect with uniform and nonuniform polarization distributions, were predicted (and subsequently discovered) within the framework of the Landau theory of secondorder phase transitions. The Landau theory modified with allowance made for the Landau potential terms of higher than the fourth degree in the components of the order parameter has been successfully used in describing variations in properties of materials, their structure, and phase diagrams, which are characteristic of firstorder phase transitions and phase transition sequences described by a specified Landau order parameter. Although the Landau theory has been employed to advantage, a number of problems arise when this theory is modified for the purpose of describing the variations observed in properties of materials upon first-order phase transitions or sequences of phase transitions induced by a one order parameter. Some of these problems will be discussed in the present paper.
Within the physical representation of the order parameter in the phenomenological theory of structural phase transitions, it is common practice to distinguish phase transitions of the order–disorder and displacive types. In the case of order–disorder phase transitions, the order parameter components are considered to be quantities proportional to symmetric li
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